U.S. Pat. Nos. 4,907,399, and 5,329,745, the teachings of which are incorporated by reference, illustrate typical packaging machines in which the packages produced consist of trays filled with poultry or other products which are wrapped with a polymeric film capable of being heat sealed. A particular film capable of being heat sealed is referred to as a shrink-wrap film. In a typical method of wrapping packages of this kind, each tray with its products is enclosed in a tube formed of film whose longitudinal edges are first sealed to each other parallel to the axis of the tube so as to produce what is called a longitudinal seal. In a succeeding stage of wrapping, the film tube is both sealed and parted along a selected line at the leading and trailing end of the tray to produce what are called the transverse end seals. U.S. Pat. Nos. 5,063,327 and 5,421,139 illustrate such longitudinal and end seals, the teachings of which are also incorporated by reference.
Longitudinal seals are typically formed by feeding the longitudinal opposed edges of the film between a pair of heated rollers as illustrated in U.S. Pat. No. 5,329,745. In this regard, it has been observed that when longitudinal seals are formed in this manner with film and particularly with shrink-wrap film, the longitudinal seals are generally found to be reliable. It has also been observed that when longitudinal seals are formed by pressing the edges of film between heated rollers, a different kind of sealing action takes place than when end seals are formed by engaging the surfaces of two sealing bars as taught in U.S. Pat. No. 5,3292,745. The end seals produced are typically not as reliable as the longitudinal seals. This difference in seal strength is particularly true in the case of shrink-wrap film when the end seals are formed by means of conventional sealing bars.
While not thoroughly understood, it is believed that the relatively poor quality of end seals is due to the fact each end seal incorporates an end of a previously formed longitudinal seal. The invention recognizes that a factor contributing to the relatively poor quality of end seals may reside in the differences in physical contact which occurs when pulling edges of film through heated rollers as compared to pressing the film between two heated bars.
Thus, the object of the invention becomes that of providing an improved apparatus and method for forming seals with film and particularly with regard to forming end seals with shrink-wrap film. Other objects will become apparent as the description proceeds.